The present invention relates to a multi-color image-forming material and a multi-color image-forming process for forming a full-color image having a high resolution using laser beam. More particularly, the present invention relates to a multi-color image-forming material and a multi-color image-forming process useful for the preparation of a color proof (DDCP: direct digital color proof) or mask image in the art of printing by laser recording from digital image signal.
In graphic art, a printing plate is made from a set of color separation films prepared from a color original through a lithographic film. In general, in order to check error in the color separation step or necessity for color correction before the final printing (actual printing), a color proof is prepared from the color separation films. A color proof is required to realize a high resolving power allowing a high reproducibility of halftone image or a high step stability. In order to obtain a color proof approximating the actual printed matter, the color proof is preferably made of the material to be used in the actual printed matter, e.g., printing paper as a substrate and pigment as a colorant. It is extremely desirable that the color proof be prepared by a dry process in the absence of developer.
As a dry process for the preparation of a color proof, a recording system for preparing a color proof directly from a digital signal has been developed with the recent spread of an electronizing system in prepressing step. Such an electronizing system is particularly adapted for the preparation of a high quality color proof and normally reproduces a halftone image having a precision of not lower than 150 lines/inch. In order to record a high quality proof from a digital signal, laser beam, which can be modulated with a digital signal and can be converged to form a fine recording beam, is used as a recording head. To this end, it is necessary that an image-forming material be developed which exhibits a high recording sensitivity to laser beam and a high resolving power allowing reproduction of a high precision halftone.
As an image-forming material to be used in the transfer image forming process using laser beam there has been known a hot-melt transfer sheet comprising a light-to-heat conversion layer which absorbs light beam to generate heat and an image-forming layer having a pigment dispersed in a hot-melt wax, binder or the like provided in this order on a support (Japanese Patent Application (Laid-Open) No. 1993-58045). In the image forming process using such an image-forming material, heat generated in the laser beam-irradiated area on the light-to-heat conversion layer causes the image-forming layer corresponding to that area to be melted and transferred to the image-receiving sheet laminated on the transfer sheet to form a transfer image on the image-receiving sheet.
Japanese Patent Application (Laid-Open) No. 1994-219052 discloses a heat transfer sheet comprising a light-to-heat conversion layer containing a light-to-heat conversion material, a heat-peeling layer having a thickness as very small as 0.03 xcexcm to 0.3 xcexcm, and an image-forming layer containing a colorant provided in this order on a support. When this heat transfer sheet is irradiated with laser beam, the adhesion between the image-forming layer and the light-to-heat conversion layer, which are bonded to each other with the heat-peeling layer provided interposed therebetween, is lowered to form a high precision image on the image-receiving sheet laminated on the heat transfer sheet. The image forming process using the heat transfer sheet involves so-called xe2x80x9cablationxe2x80x9d. In some detail, a phenomenon is used that the area which has been irradiated with laser beam is subject to decomposition and vaporization of a part of the heat-peeling layer that weakens the adhesion between the image-forming layer and the light-to-heat conversion layer, causing the image-forming layer on the area to be transferred to the image-receiving sheet laminated on the heat transfer sheet.
These image forming processes are advantageous in that a printing paper comprising an image-receiving layer (adhesive layer) provided therein may be used as an image-receiving sheet material and a multi-color image can be easily obtained by sequentially transferring images having different colors onto the image-receiving sheet. In particular, the image-forming process using ablation is advantageous in that a high precision image can be easily obtained and is useful for the preparation of a color proof (DDCP: direct digital color proof) or a high precision mask image.
With the progress of DTP environment, CTP (Computer to Plate) system has been needed more for DDCP process proof than for proof sheet or analog process proof because it requires no step of withdrawing intermediate film. In recent years, a large-sized DDCP having a high quality, a high stability and an excellent coincidence with desired printed matter has been desired.
A laser heat transfer process allows printing with a high resolution. A laser heat transfer process has heretofore been effected in various processes such as (1) laser sublimation process, (2) laser ablation process and (3) laser melt process. However, all these processes were disadvantageous in that the resulting recorded halftone is not sharp. In some detail, the laser sublimation process (1) involves the use of a dye as a colorant and thus is disadvantageous in that the approximation to desired printed matter is insufficient. This process also involves the sublimation of a colorant and thus is disadvantageous in that the resulting halftone has a blurred contour, giving an insufficient resolution. On the other hand, the laser ablation process (2) involves the use of a pigment as a colorant and thus provides a good approximation to desired printed matter. However, this process involves the scattering of a colorant and thus is disadvantageous in that the resulting halftone has a blurred contour, giving an insufficient resolution as in the laser sublimation process. Further, the laser melt process (3) involves the flow of molten material and thus is disadvantageous in that the resulting image has no clear contour.
Moreover, when a heat transfer sheet is used particularly for color proof, it is necessary that the thickness of the image-forming layer be raised to provide the image transferred to printing paper with a required reflection OD. As a result, the heat capacity of the image-forming layer increases, causing the deterioration of the recording sensitivity and resolving power of the system.
Japanese Patent Application (Laid-Open) No. 1996-300829 and Japanese Patent Application (Laid-Open) No. 1996-300830 disclose a process which comprises controlling the color power index of carbon black to not greater than 120 or not greater than 125 to obtain a transfer image having a sufficient blackness. It is described that the coloring power index is a value determined relative to that of standard black as 100 according to ASTM N-440 and is more preferably from 30 to 100.
However, even the use of such an index gave no solution to the foregoing problems.
It is therefore an object of the present invention to provide a large-sized DDCP having a high quality, a high stability and an excellent coincidence with desired printed matter. In some detail, the present invention has the following objects:
1) The heat transfer sheet can withstand comparison of pigment colorant with desired printed matter free from the effect (i.e., the influence) of the illuminating light source and allows transfer of thin colorant film resulting in the provision of dots with an excellent sharpness and stability;
2) The image-receiving sheet can securely receive the image-forming layer of laser energy heat transfer sheet in a stable manner;
3) An image can be transferred to printing paper and a close description of texture or accurate reproduction of paper white (high key portion) can be made according to a basis weight of at least 64 to 157 g/m2 as in art (coated) paper, matted paper, slightly coated paper, etc.;
4) An extremely stable transfer peelability can be obtained.
It is another object of the present invention to provide a multi-color image-forming material and a multi-color image-forming process which can form an image having a good quality and a stable transfer density on an image-receiving sheet even when laser recording is effected with a multiple laser beam having a high energy under different temperature and humidity conditions.
In particular, in order to obtain a high color proof, it is important to attain a good approximation to desired printed matter taking into account its purpose. In other words, it is important that the color hue of color proof is substantially the same as that of desired printed matter. It is also necessary that the change of visual appreciation of colors of color proof under different illuminating light sources, e.g., from fluorescent lamp and incandescent lamp be the same as that of desired printed matter.
It is therefore a further object of the present invention to provide a multi-color image-forming material which can withstand comparison of pigment colorant with desired printed matter free from the effect (i.e., influence) of the illuminating light source and thus can provide a recorded image excellent in approximation to desired printed matter.
It is a still further object of the present invention to provide a multi-color image-forming material which exhibits a high recording sensitivity and resolving power and can provide a heat transfer image having an invariably high reflection density (ODr).
It is a still further object of the present invention to provide a multi-color image-forming material which can provide a heat transfer image having an invariably good resolution.
These and other objects of the present invention will become apparent from the following detailed description and examples.
These and other objects of the present invention will become apparent from the following detailed description and examples.
These objects of the present invention are accomplished by the following as pets (1) to (32) of the present invention.
(1) A multi-color image-forming material comprising image-receiving sheets each having an image-receiving layer and heat transfer sheets for at least four colors, including yellow, magenta, cyan and black, each having at least a light-to-heat conversion layer and an image-forming layer on a support, the heat transfer sheets and the image-receiving sheets being respectively laminated such that the image-forming layer of the heat transfer sheet and the image-receiving layer of the image-receiving sheet are opposed to each other, whereby the irradiation with laser beam causes the area irradiated with laser beam on the image-forming layer to be transferred onto the image-forming layer in the image-receiving sheet to effect image recording, wherein the thickness of the image-forming layer in the heat transfer sheets is from 0.01 xcexcm to 1.5 xcexcm and the width of lines in laser-transferred image is from 0.8 to 2.0 times a half of the half-width (i.e., the half width at half maximum: HWHM) of the distribution in the direction of subsidiary scanning of the integration of the binary energy distribution of laser beam spot in the direction of main scanning.
(2) The multi-color image-forming material as described in the above item (1), wherein the heat transfer sheets are a yellow heat transfer sheet the maximum absorbance (xcexmax) of which in spectral distribution falls within a range of from 380 nm to 460 nm, a magenta heat transfer sheet the maximum absorbance (xcexmax) of which in spectral distribution falls within a range of from 540 nm to 600 nm, a cyan heat transfer sheet the maximum absorbance (xcexmax) of which in spectral distribution falls within a range of from 610 nm to 730 nm and a black heat transfer sheet.
(3) The multi-color image-forming material as defined in the above item (2), wherein the half-width measured when the maximum absorbance (xcexmax) is 1.0 is from 90 nm to 160 nm for the yellow heat transfer sheet, from 40 nm to 130 nm for the magenta heat transfer sheet and from 90 nm to 160 nm for the cyan heat transfer sheet.
(4) The multi-color image-forming material as defined in the above item (1), wherein the change of xcex94E measured with D65 or A as a light source is not greater than 2.0 for the cyan heat transfer sheet supposing that xcex94E is the color difference between the color hue (L1*a1*b1*) and the desired color hue (L2*a2*b2*) of the image-forming layer represented by the following equation:
xcex94E{(L1*xe2x88x92L2*)2+(a1*xe2x88x92a2*)2+(b1*xe2x88x92b2*)2}0.5
(5) The multi-color image-forming material as defined in the above item (4), wherein xcex94E of the cyan heat transfer sheet is not greater than 15.0.
(6) The multi-color image-forming material as defined in the above item (1), wherein the change width of xcex94E measured with D65 or A as a light source is not greater than 1.5 for the magenta heat transfer sheet supposing that xcex94E is the color difference between the color hue (L1*a1*b1) and the desired color hue (L2*a2*b2*) of the image-forming layer represented by the following equation:
xcex94E={(L1*xe2x88x92L2*)2+(a1*xe2x88x92a2*)2+(b1*xe2x88x92b2*)2}0.5
(7) The multi-color image-forming material as defined in the above item (6), wherein xcex94E of the magenta heat transfer sheet is not greater than 16.0.
(8) The multi-color image-forming material as defined in the above item (1), wherein the change width of xcex94E measured with D65 or A as a light source is not greater than 2.0 for the yellow heat transfer sheet supposing that xcex94E is the color difference between the color hue (L1*a1*b1) and the desired color hue (L2*a2*b2*) of the image-forming layer represented by the following equation:
xcex94E={(L1*xe2x88x92L2*)2+(a1*xe2x88x92a2*)2+(b1*xe2x88x92b2*)2}0.5
(9) The multi-color image-forming material as defined in the above item (8), wherein xcex94E of the yellow heat transfer sheet is not greater than 5.0.
(10) The multi-color image-forming material as defined in the above item (1), wherein the value X obtained by dividing the reflection optical density (ODr) of the image-forming layer constituting the yellow heat transfer sheet comprising at least one yellow organic pigment in the image-forming layer measured through a blue filter by the thickness (unit: xcexcm) of the image-forming layer is not smaller than 1.6.
(11) The multi-color image-forming material as defined in the above item (10), wherein the value X is not smaller than 2.0.
(12) The multi-color image-forming material as defined in the above item (1), wherein the value X obtained by dividing the reflection optical density (ODr) of the image-forming layer constituting the magenta heat transfer sheet comprising at least one magenta organic pigment in the image-forming layer measured through a green filter by the thickness (unit: xcexcm) of the image-forming layer is not smaller than 1.6.
(13) The multi-color image-forming material as defined in the above item (12), wherein the value X is not smaller than 3.0.
(14) The multi-color image-forming material as defined in the above item (1), wherein the value X obtained by dividing the reflection optical density (ODr) of the image-forming layer constituting the cyan heat transfer sheet comprising at least one cyan organic pigment in the image-forming layer measured through a red filter by the thickness (unit: xcexcm) of the image-forming layer is not smaller than 2.0.
(15) The multi-color image-forming material as defined in the above item (14), wherein the value X is not smaller than 2.9.
(16) The multi-color image-forming material as defined in the above item (1), wherein the value X obtained by dividing the reflection optical density (ODr) of the image-forming layer constituting the black heat transfer sheet comprising at least one black carbon in the image-forming layer measured through a visual filter by the thickness (unit: xcexcm) of the image-forming layer is not smaller than 2.0.
(17) The multi-color image-forming material as defined in the above item (16), wherein the value X is not smaller than 2.7.
(18) The multi-color image-forming material as defined in the above item (1), wherein the ratio of the optical density (OD) of the image-forming layer in the various heat-transfer sheets to the thickness of the image-forming layer is not smaller than 1.50, the recording area of multi-color image in the various heat transfer sheets has a size of 515 mmxc3x97728 mm, the resolution of the transferred image is not smaller than 2,400 dpi, the image-forming layer in the heat transfer sheets each comprise a polymer pigment dispersant and/or phosphoric acid ester-based pigment dispersant incorporated therein, and the polymer pigment dispersant is a copolymer or polymer blend comprising ((C2H5)2Nxe2x80x94(CH2)zxe2x80x94Oxe2x80x94) (in which z represents an integer of 2 or 3), ethylene glycol and propylene glycol at a ratio of 1:X:Y in which X and Y represent a number of from 10 to 20 and from 25 to 40, respectively.
(19) The multi-color image-forming material as defined in the above item (1), wherein the heat transfer sheets each comprise an organic pigment and/or carbon black incorporated as a colorant in the image-forming layer and the organic pigment and/or carbon black is monodisperse and has a particle diameter variation coefficient of not greater than 50%.
(20) The multi-color image-forming material as defined in the above item (19), wherein the organic pigment and/or carbon black has an average particle diameter of from 50 nm to 1,000 nm.
(21) The multi-color image-forming material as defined in any one of the above items (1) to (20), wherein the transferred image has a resolution of not smaller than 2,400 dpi.
(22) The multi-color image-forming material as defined in the above item (21), wherein the transferred image has a resolution of not smaller than 2,600 dpi.
(23) The multi-color image-forming material as defined in any one of the above items (1) to (22), wherein the ratio of the optical density (OD) of the image-forming layer in the various heat transfer sheets to the thickness of the image-forming layer is not smaller than 1.50.
(24) The multi-color image-forming material as defined in the above item (23), wherein the ratio of the optical density (OD) of the image-forming layer in the various heat transfer sheets to the thickness of the image-forming layer is not smaller than 1.80.
(25) The multi-color image-forming material as defined in the above item (24), wherein the ratio of the optical density (OD) of the image-forming layer in the various heat transfer sheets to the thickness of the image-forming layer is not smaller than 2.50.
(26) The multi-color image-forming material as defined in any one of the above items (1) to (25), wherein the image-forming layer in the various heat transfer sheets and the image-receiving layer in the image-receiving sheets each exhibit a contact angle of from 7.0xc2x0 to 120.0xc2x0 with respect to water.
(27) The multi-color image-forming material as defined in any one of the above items (1) to (22), wherein the ratio of the optical density (OD) of the image-forming layer in the various heat transfer sheets to the thickness of the image-forming layer is not smaller than 1.80 and the image sheets each exhibit a contact angle of not more than 860 with respect to water.
(28) The multi-color image-forming material as defined in any one of the above items (1) to (27), wherein the recorded area of multi-color image has a size of 515 mmxc3x97728 mm.
(29) The multi-color image-forming material as defined in the above item (28), wherein the recorded area of multi-color image has a size of 594 mmxc3x97841 mm.
(30) The multi-color image-forming material as defined in any one of the above items (1) to (29), wherein the image-forming layer comprises a pigment and an amorphous organic polymer having a softening point of from 40xc2x0 to 150xc2x0 incorporated therein each in an amount of from 20% to 80% by mass (i.e., by weight) and has a thickness of from 0.2 xcexcm to 1.5 xcexcm.
(31) A multi-color image-forming process which comprises laminating an image-receiving sheet as defined in any one of the above items (1) to (30) with each of at least four different color heat transfer sheets as defined in any one of the above items (1) to (30) such that the image-forming layer of the heat-transfer sheet and the image-receiving layer of the image-receiving sheet are opposed to each other, irradiating the laminate with laser beam, and then transferring the laser beam-irradiated area on the image-forming layer onto the image-receiving layer in the image-receiving sheet to effect image recording, wherein the image-forming layer on the laser beam-irradiated area is transferred to the image-receiving sheet in the form of thin film.
(32) The multi-color image-forming process as defined in the above item (31), wherein when irradiated with laser beam, the light-to-heat conversion layer softens so that the image-forming layer on the light-to-heat conversion layer is pushed up and transferred to the image-receiving sheet in the form of thin film.
(33) The multi-color image-forming process as defined in the above item (1), wherein the thickness of the image-forming layer in the heat transfer sheets is from 0.01 xcexcm to 0.9 xcexcm.
(34) The multi-color image-forming process as defined in the above item (1), wherein the width of lines in laser-transferred image is from 0.8 to 1.7 times a half of the half-width (i.e., the half width at half maximum: HWHM) of the distribution in the direction of subsidiary scanning of the integration of the binary energy distribution of laser beam spot in the direction of main scanning.
(35) The multi-color image-forming process as defined in the above item (1), wherein the width of lines in laser-transferred image is from 0.8 to 1.2 times a half of the half-width (i.e., the half width at half maximum: HWHM) of the distribution in the direction of subsidiary scanning of the integration of the binary energy distribution of laser beam spot in the direction of main scanning.